Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
As the pixels become smaller, the surface area that can receive incident light is also reduced. The pixel typically has a light-sensing element, such as a photodiode, which receives incident light and produces a signal in relation to the amount of incident light. Because of the small size of the light-sensing element, it is important that as much incident light is captured by the light-sensing element. One major source of incident light loss occurs as a result of reflection at the photodiode (the silicon surface) to oxide (SiO2) interface. At this interface, a substantial amount of light is reflected with the consequential reduction in photodiode responsivity and quantum efficiency.